The problem addressed by the invention is the design of the downlink of a multi-access wireless network system utilizing full frequency and spatial diversity for each intended user while taking advantage of error correction coding techniques that employ iterative decoding architectures. In particular, the signaling content sent to all users by the Access-Point (AP) occupies the entire physical bandwidth during the same period of transmission (i.e. spread across all sub-carriers of an OFDM symbol).
The term AP is general and applies to many fields. In the particular case of wireless telecommunications, APs are also referred to as base stations.
Previous work has employed wireless network systems based on Code-Division-Multiple-Access (CDMA); in particular, Direct-Sequence CDMA (DS-CDMA) communication systems. FIG. 1 illustrates a typical DS-CDMA system in which each user is assigned a chip sequence (also referred to as a spreading code), wi for i=1, 2, . . . , M (where M is the length of the sequence and also the maximum number of users), that is ideally pair-wise orthogonal to all other spreading codes that may be used by other users, i.e.
            w      i      T        ⁢          w      j        =      {                                        1                                                              for                ⁢                                                                  ⁢                i                            =              j                                                            0                                                              for                ⁢                                                                  ⁢                i                            ≠              j                                          .      
Then, the AP based on DS-CDMA modulates each user's data with the corresponding spreading code in a scalar fashion using points from a constellation constructed from the respective user's data and adds chip-by-chip the results prior to transmission to all users included in this operation (see FIG. 1). The AP transmits this sequence of sums one chip period at a time (spreading the users' data over time) in synchronized periodic intervals to all users. Each user then uses a match filter (known by those skilled in the art) based on its assigned chip sequence to remove the modulated spreading codes of all other users. This is possible because each user's signal space exists in an orthogonal dimension. Using the output of the match filter, the user then decodes the information modulated in the transmitted constellation point.
FIG. 1A illustrates in simplified form a typical base station, in which a set of modules 110-1 to 110-M encodes the data being transmitted by the user, using a code such as an LDPC turbo code that has an error-correcting feature that permits the receiving station to correct for errors generated in the channel.
Next in sequence, the codewords that are the output of each encoder are optionally interleaved (on a bit or symbol basis) in units 120-1 to 120-M in order to spread the data out in time and/or space and thereby to increase the diversity of the signals.
The interleaved data are used to modulate frequency domain symbols in units 130-1 to 130-M, the output of which are modulated with the spreading codes w1-wM.
The M data streams are then added in adder 150 and broadcast on one or more antennas 155.
FIG. 1B illustrates graphically that the data for each user is spread over the entire spectrum available for the system, being separated from one another by the orthogonality of the spreading codes.
The plus sign in unit 140 of FIG. 1B indicates that the data for different users is combined and processed at the same time; data for an individual user is transmitted sequentially one chip period at a time.
The result, illustrated graphically on the right of FIG. 1B, is that the contents of a point in the Cartesian frequency-time graph will contain data for all users.
From a temporal viewpoint, the AP sends data to all users at the same time. From a spectral viewpoint, each user's spectral content occupies the entire physical bandwidth (inverse of a chip period) leaving the AP's transmitter. Thus, in DS-CDMA all users assigned a distinct spreading code occupy all frequencies at the exact time during the AP transmission. In existing DS-CDMA systems, the spreading occurs across time for a given carrier frequency.
An alternate prior art application of orthogonal spreading codes is placing the modulated chip sequence across sub-carriers in systems employing OFDM signaling (a.k.a. Multi-Carrier-CDMA (MC-CDMA)). Again, all assigned users exist simultaneously across the same physical frequencies and time epochs but in orthogonal signal spaces. Nevertheless, this prior art and variations of such also separates users using assigned orthogonal spreading codes.
Another prior approach is Orthogonal Frequency Division Multiple Access (OFDMA), illustrated in FIG. 2. This approach partitions sub-carriers of an OFDM signaling system into groups of adjacent (or possibly non-adjacent) sub-carriers where each mobile terminal is assigned a group (or groups) for the purpose of multiple-access. Thus, each user experiences frequency diversity only within its assigned groups and not the entire available frequency bandwidth (unless its group consists of the entire bandwidth).
FIG. 2 shows a set of modules similar to those of FIG. 1. User data enter on separate lines in the left of the Figure and are encoded in a set of modules 210-1,-210-M.
The encoded output is interleaved in modules 220-1-220-M. The interleaved output is used to modulate frequency-domain symbols in modules 230-1,-230-M.
The modulated frequency-domain data pass to a process unit 240 that performs an inverse Fourier transform. The multiple arrows between units 230-I and unit 240 indicate that a number of subchannels separated in frequency are jointly processed with an inverse Fourier transform, resulting in a composite time domain signal output from FT unit 240.
The time domain signal is optionally combined with a cyclic prefix for purposes of utilizing the circular convolution property of the Discrete Fourier Transform (DFT) pair (of which the Fast Fourier Transform (FFT) pair is a special case) to maintain sub-channel orthogonality in the presence of frequency-selective multipath propagation.
FIG. 2B illustrates graphically the process, in which the available spectrum is divided in frequency with an Nth user having data on one or more subchannels.
As with FIG. 1B, the transmitted signal at a given time contains data for multiple users. On the right side, the diagram differs from that of FIG. 1B indicating that each user has a separate portion of the spectrum.